The present invention relates to a method for rework of a printed circuit board; more particularly, the method relates to a method for removing an unwanted electrical printed circuit board conductor from a printed circuit board being reworked.
The problem of repair and rework involving changes to printed circuit board wiring becomes more difficult as printed circuit boards become increasingly miniaturized, and increasingly formed of multiple layers. Miniaturization of printed circuit boards leads to extremely narrow conductor paths, with very close conductor spacing. Constructing printed circuit boards in multiple layers compounds the problem of accessing conductors embedded several layers deep into the circuit board.
In the prior art, removal of existing printed circuit board conductors during the rework process involved cutting the unwanted conductors at particular locations utilizing a sharpened knife, or utilizing specialized cutting tools. Because of the close tolerancing of adjacent conductors on printed circuit boards, specialized tools such as dental drills and precision cutting wheels have been used for this purpose. In some cases, for example, it has been possible to cut through an underlayer conductor with a dental drill without harming either adjacent conductors or conductors on adjacent layers. However, the rework process in this regard has become much more difficult as the density of circuit board wiring is increased, and the miniaturization of the circuits has increased.
Of course, it is possible to custom design printed circuit boards with the idea of subsequently and intentionally removing conductors, as by way of custom designing conductors having special fusible links built therein. However, this practice relates to the problem of designing custom circuit boards which may subsequently be customized for special purposes, and does not relate to the more general problem of reworking conventionally designed circuit boards. In a conventionally designed circuit board the multiple layers of conductors are laid out to accomplish a specific circuit design function, and no attention has been given to any need for subsequently removing these conductors. Therefore, when rework is attempted on such circuit boards there are no built-in design features to facilitate the rework, and the only tools available for such rework have been precision cutting tools.
It would be an advantage to have a method for reworking printed circuit board conductive paths without the necessity of imposing certain design requirements on the construction of the initial printed circuit board, or without the need to employ precision cutting instruments. The present invention overcomes these difficulties by providing a method for selectively deleting printed circuit board conductors without regard to the techniques under which the circuit board was initially constructed.